The Clock That Changed the World

In the 18th Century, navigating across the seas was fraught with danger and error. With no practical method of determining longitude, sailors had no accurate means of pinpointing their position at sea. Ships had sailed forth only to blunder off course and never return. Sailors had died by the thousands. And, mistakes had jeopardized trade endeavours and dearly cost the governments of the day. A reliable means of finding longitude was becoming a matter of pressing concern.

Successful coastal navigation depended upon an accurate determination of both latitude and longitude. Determination of latitude had comparatively been a much less problematic affair, rendered an effortless task through the use of instruments such as compasses, cross-staffs, and sextants. The cross-staff had helped find the latitude by measuring the Meridian altitude of the sun or the stars. It consisted of a movable wooden cross-piece that was fixed to the end of a long rod. One end of the instrument would be held close to the eye and the cross-piece would then be adjusted, such that its ends subtended an angle formed by the distance of the two objects being measured, the celestial object and the horizon. The subtended angle would then be read-off the corresponding markings on the instrument’s scale. The cross-staff, thus, allowed the measurement of the angle of the celestial object relative to the horizon.

This was perhaps one of the reasons why latitude was not entirely problematic to figure out. The horizon pitches in the same way with respect to the sun, moon, and stars. Longitudinal lines however rotate with the Earth. To establish one’s longitude at sea, it was important to determine the time it had been aboard the ship plus the corresponding time at another place of known longitude. Since there are 360 meridians of longitude and Earth rotates once every 24 hours, the Earth turns 15 degrees of longitude every hour. Accordingly, a time difference between the ship and the home point correlates to the longitude east or west. At sea, the seaman synchronizes his local time at noon, when the sun perches at the highest point in the sky, and every one hour difference between the seaman and the home point corresponds to 15 degrees of longitudinal separation. A miscalculation of time could translate to a longitude error of thousands of miles. As such, a durable clock, that was accurate enough not to lose its time or degrade in use as an extended voyage aboard a sailing vessel proceeded amid temperature changes and rolling waves, was in much need. It had not been possible with the concomitant technologies of the time, however.

In 1675, the Royal Observatory was established in Greenwich, England by King Charles II ‘for the practical purpose of accurate navigation out of sight of land’. It sought to solve this persisting problem by measuring the positions and motions of stars in the sky. It was hoped that accurate predictions of star positions would give sailors a concrete idea of their longitude. Stars were to be mapped at Greenwich, at which point sailors could use them to compare them to the position of stars at their location. Such a method, however, had proven rather difficult to follow and problems ensued.

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The Royal Observatory of Greenwich

Indeed, 30 years later, in 1707, disaster struck when four British warships misjudged their position and crashed into the Isles of Scilly. More than 2000 men had died in one of the most notorious navigational accidents in history. The problem of longitude was now a matter of supreme national importance. In response, the British parliament enacted the Longitude Act and offered a reward of 20,000 pounds (12 million dollars in today’s money) to anyone who could “devise a method of determining a ship’s longitude”.

The Scilly naval disaster of 1707, a result of an inability to determine position

The prize would not be claimed by the astronomers of the day but rather by a humble self-taught English carpenter, a Mr. John Harrison. The ambitious Mr. Harrison had grown increasingly interested by the challenge. He had shown an innate fascination with clocks from an early age. A year earlier, at the age of 20, he had already completed his first long-case pendulum clock, made entirely out of wood.

In 1730, Harrison visited London to present Astronomer Royal and head of the Royal Observatory, Sir Edmund Halley, with plans to build a marine timepiece designed not to degrade aboard any ship in all sorts of atmospheric conditions. The astronomers at the Board of Longitude were rather dismissive of Harrison’s ideas of a mechanical solution. Halley, however, was more accepting and referred Harrison to see eminent watchmaker George Graham who was so impressed that he became his benefactor for the proposed clock.

John Harrison, Yorkshire carpenter

In the next five years, Harrison would complete his first model of the marine chronometer, Harrison No. 1, or H1. At four feet high, it was a huge clock that had a remarkable mechanism to counterbalance the effects of motion. It kept time to about 8 seconds per day. A trial was conducted the following year in 1736, where the clock was sent on a voyage aboard a ship to Lisbon to test its accuracy. The clock had performed well and accurately guided the ship along the trip, however not accurately enough to win the prize. Harrison thought he could do better and at his request, he received funds from the Board to continue his work on a second model.

The H1 Clock

The improved model, the H2, was almost just as humongous, and had a refined temperature compensating mechanism. Unsatisfied with its design, Harrison requested more funds and for the next two decades, he would continue working on a further improved model, the H3.  The H3 was half the size  of H1 and had an innovative, temperature-sensitive bi-metallic strip and novel bearing technologies. The H3 was so accurate that on average, it kept time to about 2 seconds per day. The Board of Longitude, however, refused to award the prize and demanded further modifications.

The H2 Clock

Astronomers, however, were not idle and in the coinciding years, Naval officer John Campbell had redesigned the octant to develop the sextant,  which very accurately measured the latitude (to the +/- 0.01 of a degree). It used two mirrors to measure the angle of the North Star (very conveniently perched directly over the North Pole) with respect to the horizon. The altitude of the North Star would just be the latitude. Campbell had proposed using the Royal Observatory’s star map and the sextant, as a reliable method of determining longitude. If successful, it would save Parliament the trouble of having to pay out the 20,000 pounds.

The Harrison H3 Clock

This hindered any further trial voyages for Harrison’s models and he had to wait three years, until his next improved model, the H4, a radical departure from his previous ones, was to be finally put to the test. It was tested on two voyages to the West Indies in 1761 and 1764. It was a miniature 3 pound pocket watch and was so accurate that on the first trip, it was on error by only 5.1 seconds, corresponding to an error of less than 1.25 nautical miles. On the second trip, it was found to be on error by 39 seconds. Both were very excellent results.

The H4 Clock


The Board still refused to award Harrison the prize. The newly appointed Astronomer Royal, Neville Maskelyne, was a proponent of the lunar distance method, and had hoped to win the prize himself. A fierce battle ensued between both men in the coming years. Harrison appealed to King George III for assistance. By that time, he had designed a fifth model, the H5. The king tested Harrison’s H5, at his own observatory, and found it to be accurate to one third of a second per day, over a six month period. It was not until the king’s intervention and an act passed by Parliament that Harrison finally received his money in 1773, 3 years short of his death.

Harrison’s clock would become the most elegant chronometer ever built. It was much more reliable than the Royal Observatory’s star maps and would transform navigation, transport, communication, and trade in the upcoming centuries. A very expensive instrument, it would, nonetheless, become more affordable by the 19th century. The HMS Beagle, which carried Charles Darwin around the world for five years, had 22 chronometers aboard.


Landes, DS. 1983. Revolution in Time: Clocks and the Making of the Modern World. Cambridge, MA: Belknap Press of Harvard University Press.

May, WE. 1973. A History of Marine Navigation, Norton, New York.

Quill, H. 1966. John Harrison: the Man Who Found Longitude. London: Baker.

Sobel, D. 1995. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Penguin Books.

Whittle, ES. 1984. The Inventor of the Marine Chronometer: John Harrison of Foulby. Wakefield Historical Publications.



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